Kidney injury by ischemia leads to acute renal failure (ARF), a major kidney disease associated with high mortality. While the development of ARF involves multiple factors and may proceed in several phases, it is ultimately precipitated by sublethal and lethal damage to renal tubular cells. The long-term goal of our research is to delineate the apoptotic mechanism of tubular cell injury and identify novel strategies for the prevention and treatment of ARF. During the last grant period, we and others have shown the involvement of tubular cell apoptosis in ischemic and nephrotoxic renal injury. Importantly, these studies have demonstrated a pivotal role for Bax/Bak- mediated mitochondrial damage. We have further revealed a striking morphological change of mitochondria during tubular cell apoptosis, i.e. mitochondrial fragmentation. Inhibition of mitochondrial fragmentation prevents mitochondrial damage and apoptosis. Despite these findings, critical questions remain: 1) how is Bax activated? 2) how is mitochondrial fragmentation induced and regulated? Endophilin B1 (Endo-B1) was originally identified as a Bax-interacting protein by yeast two hybrid screening. Our preliminary studies have demonstrated an early Endo- B1 activation and translocation to mitochondria during ischemic renal injury. Importantly, knockdown of Endo-B1 suppresses mitochondrial fragmentation and tubular cell apoptosis. The overall objective of this project is to elucidate the regulation of Bax and mitochondrial morphological dynamics by Endo-B1, and determine the role of Endo-B1 in tubular cell apoptosis during ischemic renal injury. We hypothesize that upon apoptosis induction, Endo-B1 interacts with Bax, leading to their translocation to mitochondria. In mitochondria, Endo-B1 collaborates with fission-fusion proteins to fragment the organelles, facilitating Bax/Bak oligomerization and formation of pathological pores, resulting in the release of apoptotic factors. We propose three Specific Aims to test this hypothesis: 1) determine the role of Endo-B1 in Bax activation and analyze Endo-B1/Bax interactions during renal cell apoptosis; 2) delineate the role and regulation of Endo-B1 in mitochondrial fragmentation during renal cell injury; and 3) use a newly developed knockout mouse model to determine the role of Endo-B1 in Bax activation and mitochondrial fragmentation during ischemic renal injury in vivo. Completion of the research is expected to significantly advance the mechanistic understanding of mitochondrial injury during apoptosis. In addition, it may lead to the development of novel strategies for the prevention and treatment of ischemic renal failure.